1. Field of the Invention
The present invention relates to a corrosion-protected tension member of steel composed of a plurality of individual wires twisted into a strand. The invention also relates to a wedge-type anchoring system with such a tension member of steel.
2. Description of the Related Art
When building structures, especially structures subjected to environmental influences, the protection of steel components against corrosion has always posed problems. Reinforcement elements for reinforced steel are substantially protected against corrosion by the adhesion or bond with the concrete surrounding the elements and prestressing members for prestressed concrete with subsequent bonding are substantially protected against corrosion by pressing cement mortar into the sheathing tubes; however, the reinforcement elements or the prestressing members become components of the structure as a result and can only be separated therefrom by destruction. More recently, regulatory agencies overseeing construction require more and more frequently that tension members, particularly prestressing members, are accessible and, thus, controllable, and possibly also exchangeable, in order to be able to detect corrosion damage early and to repair the damage.
Strands of twisted steel wires are usually used for tension members for prestressed concrete which frequently also extend outside of the cross-section of the structure, as well as especially for highly stressed prestressing members, such as, stay cables of cable-stayed bridges. Such strands usually are composed of seven high-strength steel wires, wherein six outer wires are grouped around a straight central wire. Such strands are usually anchored by means of wedge-type anchoring systems.
Many tensioning methods used in prestressed concrete constructions are based on the principle of wedge-type anchoring. When the tensile force to be anchored is applied, the wedges usually composed of several wedge-shaped sectors are pulled by the tensile force in the direction of the prestressing member axis into a conical bore in an anchor body; this procedure is called slippage. As a result, clamping forces directed radially of the prestressing member axis are produced in the wedge-shaped sectors, wherein these clamping forces block the movement of the prestressing member. A requirement for this is that the coefficient of friction between the prestressing member and the wedge is greater than the coefficient of friction between the wedge and the conical bore. In order to ensure this, the inner surfaces of the wedge-shaped sectors are usually provided with a profiling, for example, a thread which is cut into the truncated cone-shaped wedge body before the wedge body is cut into the individual wedge-shaped sectors which are subsequently hardened.
Of course, an effective corrosion protection of such steel wire strands must also include the anchoring areas. Accordingly in the case of so-called grease strands, i.e., strands which are surrounded by grease for corrosion protection and are provided with a sheathing of an extruded casing of synthetic material, for example, PE (U.S. Pat. No. 3,646,748), the corrosion protection extending over the free length of the strands must also continue into the anchoring units of the strands. This is achieved by having the ends of the PE casing removed at the ends of the strands over a certain distance extend into a chamber filled with grease (EP 0 323 285 B2). Aside from the fact that such a PE casing does not provide an absolute protection against mechanical damage, for example, when the strands are mounted, this type of corrosion protection requires a relatively complicated and, thus, expensive construction of the anchoring system.
It is also known in the art to provide reinforcement elements of steel with a coating of epoxy resin in order to protect them against corrosion. In order to anchor such strands by means of wedges, the wedges used for this purpose are those which have at the inner surface thereof a coarse toothing with dull or rounded-off tooth peaks, wherein the thickness of the coating and the height and inclination of the teeth are selected in such a way that the teeth peaks penetrate the coating and are pressed into the surface of the individual elements (DE 34 37 107 C2). As a result of the radial clamping force exerted by the wedges, the material of the coating is displaced; however, the portions of the surface of the strands not contacted by the teeth of the wedge are still coated, so that the penetration of oxygen to those areas is prevented where the wedge and the strand contact each other; in this manner, corrosion due to friction is to be prevented. However, a coating of epoxy resin does not provide an absolute mechanical protection; in addition, epoxy resin is heat-sensitive and may burn.
In accordance with another concept generally known in the art, a reinforcement element for concrete, or a strand, is protected against corrosion by surrounding the entire element over its entire length continuously by a protective metal sheathing (DE 29 44 878 A1). Similar to the grease strand described above, which is mechanically more resistant to a protective sheathing of metal, also in accordance with this proposal a protective layer and/or compensating layer is to be arranged between the surface of the reinforcement elements and the protective sheathing. As a result, a play remains between the tension member and the sheathing so that a wedge-type anchoring system poses at least some problems.
Finally, EP 0 563 735 A1 discloses a composite wire which is composed of a core of high-strength steel wire and a sheathing of longitudinally bent and longitudinally welded strip which is connected in a frictionally engaging manner to the core wire by means of a contact-type drawing process. Compared to the conventional steel wires protected against corrosion by galvanizing, by a PE casing or a coating of synthetic material, this composite wire has the advantage that the corrosion protection simultaneously produces a mechanical protection. Because of the rigid connection of the sheathing with the core wire, tensile strengths are achieved which are comparable to those of high-strength steel wires. However, this composite wire is only available as strand material.